American patent applications Ser. Nos. 08/290,883 now abandoned and 08/406,393, now abandoned the second being partly the sequel to the first one, filed on Aug. 18, 1994 and Mar. 20, 1995 (see also British patent No. 2,264,655 B published on May 24, 1995, British patent application No. 94/07,441.6 filed on Apr. 14, 1994, published international patent application WO 93/15,822 and international patent application WO 95/00,489) respectively disclose a certain number of forms of particle precipitation devices intended for separation of particles carried along in a turbulent gas stream (generally, but not necessarily, air). Reference is made to these documents in the description hereafter. The particles can be solid or liquid.
In the specific description hereunder, one of the aspects of the present invention is notably explained for separation of an oil fog contained in a turbulent air stream, i.e. ultra-fine suspended oil particles, typically droplets of a size below 0.5 micron. Such oil-laden air streams are encountered in the form of emissions of industrial machines working at high speeds.
However, it can be noted that this example of the present invention relative to the separation of oil droplets contained in an air stream is not limitative within the scope of the present invention since the invention also applies to the separation (precipitation) of other suspended liquid or solid particles, for example dust, vapour or smoke. Such a separation can be performed by one of the precipitation devices disclosed in the aforementioned patent applications and patent, by the variant of the previously disclosed precipitation devices described in the present document or by any other form of precipitation device such as an electrostatic precipitation device capable of performing the desired separation of particles contained in a gas stream.
It has been discovered that the efficiency of a particle precipitation device is substantially higher if the particles have a certain minimum size. Separation of particles with a size below one micron by means of a precipitation device is less effective or may require a costlier equipment. For example, a suitable separation of particles having a size below one micron can require a multi-stage precipitation device or a precipitation device of a disproportionate length, which will lead to an unsatisfactory result or require setting of an excessively costly equipment.
One of the objects of the present invention is to avoid these difficulties by proposing a method and a device intended to increase the size of the particles contained in the gas stream before they enter the precipitation device.
This objective is reached through agglomeration of the particles prior to their entering the precipitation device. Although the agglomeration technique described here is particularly effective when the particles initially occur in the form of a fog, it is also applicable to particles exhibiting the form of dust, vapour or smoke. Furthermore, this technique is not limited to particles whose initial size is below one micron. Nearly any size increase of the particles entering the precipitation device is advantageous. Therefore, when it is said here that this technique is aimed at agglomeration of "fine" particles, all the particles that are too small to be subjected to a fast and effective separation in a precipitation device, including the ultra-fine particles present in a fog, should also be included therein.